The present invention relates to methods for the conversion of waste plastic to lower molecular weight hydrocarbon materials, particularly valuable hydrocarbon materials such as hydrocarbon fuel materials. The present invention relates in particular to the decomposition of hydrocarbon polymers of waste plastics, which have a high molecular weights (long carbon chain lengths), to lower molecular weight hydrocarbons, particularly to hydrocarbons in the gasoline range (C7 to C11 hydrocarbons) or to hydrocarbons in the diesel fuel range (somewhat higher carbon chain length).
The production of hydrocarbon fuels (gasoline, diesel and the like) via catalyzed, and non-catalyzed thermal, decompositions of waste plastic, followed by separation and collection of the fuel product, is known, and has been known for decades. Pre-decomposition sorting and identification of the waste plastics is also well known.
The environmental benefits of producing fuel and other valuable low molecular weight hydrocarbon materials while eliminating plastic waste via the decomposition, or breaking down, of the polymer molecules of plastics to fuel-range hydrocarbons and/or other valuable hydrocarbons, has long been recognized, and has been commercialized. Some early commercial installations in Europe were short-lived for economic reasons, but commercial installations continue in Japan and other countries.
Major drawbacks or difficulties encountered in commercial-scale processes include: (a) chlorine removal when chlorine-containing polyvinyl chloride is among the plastic wastes; (b) heat gradients due to poor heat conductivity of plastics, resulting in char accumulation at heat transfer surfaces; and (c) economics, varying from high catalyst costs/consumption to high energy consumption. Further, the implementations of commercial-scale processes are also adversely impacted by the complexities of the installations required and the sophistications of their operations.
The prevailing high costs of hydrocarbon fuels and the environmental desirability of eliminating waste plastic combine to demand efficient yet simple and uncomplicated methods for achieving these goals. Governmental plastic waste elimination requirements, particularly in countries other than the U.S., apparently were significant motivations for existing commercial plastic-decomposition installations, particularly outside of the U.S. The increasing cost of hydrocarbon fuels obviously augments such incentives.
Further, the current major alternative fuel sources being heavily explored, such as for instance crop-plant biomass fuels (bio-fuels) and wind generators, have inherent drawbacks, including without limitation (a) the diversion of crop-producing resources (including arable land) from food production to fuel production, (b) the re-engineering of machinery that is often required in order to run on bio-fuels and (c) the harmful penetration into air spaces normally inhabited almost exclusively by our bird population and the documented incidents of devastation of bird populations, particularly when windmills and the like are placed along major migratory routes.